Jirka, Roberto also wrote me that he was able to work around it by hacking 
ycp.pm: 
    elsif ($reftype eq "HASH") 
    { 
        print "\$["; 
        while (my ($key, $value) = each %$val) 
        { 
            Return ($key, $quote_everything); 
            print ":"; 
# just to test 
            if ($key eq "uid") { 
               print WriteYcpString($value); 
            } 
            else { 
               Return ($value, $quote_everything); 
            } 
            print ","; # trailing comma is allowed 
        } 
        print "] "; # no "]:" 
    } 
 

Martin, the "conversion" (to octal number) is done on the way from agent (here
ag_passwd) to yast (Users.pm). It looks like I can't handle it from my code...

short test shows that ycp::Return (\%myhash, 1); could work and yast2-users is
able to read all the data (including integers) correctly. However proper testing
would be necessary.

I agree with Jiri that problem is in ag_passwd; if I use ycp::Return(\%myhash,
1), the second parameter (1) tell to ycp::Return routine to handle every values
in the hash '%myhash' quoted as a string. File ag_passwd probably need to be
changed starting from line 646 (ycp::Return (\%{$users{"local"}}) ) adding the
second parameter to quote values almost for hashes users, shadow and groups
(local and system). We should verify if uidnumbers quoted (i.e. '12345') is a
problem to the modules that are using ag_passwd.

done in yast2-pam-2.13.0